This invention relates to the manner of generating, controlling, and distributing electrical power from an electrical generator driven by an internal combustion engine. The generated electrical power is used to power computer-controlled electric motors used as the traction drive in multipurpose lightweight vehicles and equipment such as mowers, and to provide power to on-board attachments and external electrical equipment.
Lightweight vehicles exist today in numerous configurations and are purposefully built to meet the application needs related to the industry in which they are used. Typical examples of these vehicles are: Ride on Lawn Mowers; Yard and Garden Tractors; Snow Blowers; Golf Carts and Utility Carts; Traffic/Parking Police Scooters; Postal Delivery Vehicles; Airport People Movers; Airport Tarmac Shuttle Vehicles; Disabled-Person Movers; Hybrid Electric Vehicles; Go-carts; and All-Terrain Vehicles. These vehicles require a power source that is typically directly or indirectly mechanically linked to the drive wheels for traction and some vehicles are provided with a mechanical connection for powering onboard attachments and externally attached devices. Drive power trains typically have used drive axles, chain/sprocket drives, manual gear-selection transmissions, hydrostatic transmissions, differential gears, etc. in varying combinations. Steering and speed control techniques vary between the different types of vehicles. Most of the vehicles use a mechanical differential in the drive train to balance the torque applied to the driven wheels so that the wheels can rotate at different speeds when they are required to make a turn
The power sources for the listed vehicles have been either battery-powered electric motors or internal combustion motors. Both of these sources have shortcomings when they are used separately in a drive system.
Negative features of battery-powered motor driven vehicles have been the battery charge cycle, battery life, battery weight, space required for the batteries, and replacement costs of batteries. Many tasks cannot be completed without the batteries having to be recharged due to the length of operational time required or due to the batteries not being fully recharged. The charging time required can be excessive. The weight of the batteries adds to the load on the drive and a large space is required on the vehicle for mounting the batteries.
The internal combustion engine has features that detract from its use in directly driving a transmission and differential. Low output torque at low speed and decreasing torque beyond an optimum speed somewhere below maximum speed occurs in this engine. A typical engine will have a range of speeds up to 3300 RPM but torque efficiency will be maximized between 2500 and 2800 RPM. The loss of efficiency increases the thermal dissipation in the engine which causes fatiguing and failure of engine components. At low speeds, excessive vibration of the engine is also a problem. Continual operation of the internal combustion engine at its most efficient speed is desirable, but converting the fixed input speed from the engine to a variable speed output from the transmission is not efficient.
With the advent of solid-state power-switching devices such as MOSFETs (metal oxide semiconductor field effect transistors) and IGBTs (insulated-gate bipolar transistors) and microprocessors, the electronic controls for generators and motors that were very complex and expensive in the past, have become economically practical. Today, the electric generator/motor drive provides the flexibility in control and the ruggedness in assembly needed for a small electric motor-driven vehicle. Thus, an improved innovative small vehicle drive system has been developed with an electric generator driven by an internal combustion engine and an electric motor that provides high output torque up to its base speed. The generator supplies electrical power through a power control module to the motor/gearbox on each driven wheel and may provide external power through 110/120V AC and 12V outlets.
The present invention is directed towards a drive system, which integrates an electric generator; one or more electric motors and an electronic control module as a variable speed drive in either single, dual or four-wheeled traction drive configurations for a lightweight vehicle such as a mower. The generator is mechanically driven by the output shaft of an internal combustion engine to generate the electrical power for energizing the electric motors. A central computer in the electronic control module controls the output voltage of the generator and the speed and torque of each of the motors in the drive system. The speed input signal for the motors can be analog signals that come from sources such as a joystick, a potentiometer mounted on a steering wheel, control panel, foot pedal or remote location or digital signals from a digital device. Position/speed detectors on each motor and in some configurations the generator send signals back to the central computer for closed-loop control of the generator and of the motors. The generator supplies the DC voltage to the power control board for each motor as commanded by the central computer. The rotor of each motor is connected to a gearbox for speed reduction and increased torque that is applied to the wheel mounted on the output shaft of the gearbox. When the motors are not being driven, the generator may optionally supply DC power to the input of an electrical inverter that has an output to standard electric utility AC outlets that can be used to power auxiliary equipment.
This invention integrates 1) a high-efficiency switched reluctance (SR) electric generator driven by an internal combustion engine, 2) high-efficiency switched reluctance electric motors, 3) speed reduction gearboxes and 4) a power control module. The battery used for starting the internal combustion engine supplies power to the central computer and low voltage circuitry. The central computer in the power control module controls the generator output by controlling the electrical excitation to the generator from a solid-state generator power control board circuit in the power control module. The generator supplies power to each motor that directly drives a gearbox mounted to each driven wheel. In this drive configuration, the internal combustion engine can be run continually at the speed where it is most efficient.
The SR generator is driven by a pulley on its input shaft that is connected via a v-belt to a pulley on the output shaft of the internal combustion engine. The generator can also be driven directly by mechanically connecting it on the engine output shaft. The mechanical connection to the SR generator is driven by the motor at the speed that provides the highest efficiency in the engine. The electrical output of the generator is controlled by the central computer and a generator circuit in the power control module that supplies the excitation to the generator. The generator has an encoder, such as a Hall sensor or optical sensor, on the rotor that sends a position/speed signal to the power control module that must know the location of the rotor for control of commutation in the generator. The output is monitored by the power control module to determine the level of excitation required in order to maintain the correct output level. The output of the generator is electrically supplied to each of the motor power control boards in the power control module.
An inverter module can optionally be connected to the output of the generator to provide AC power for auxiliary equipment. The filtered DC input power from the generator is chopped by a semiconductor H-bridge. The switching in the H-bridge is controlled by the central computer board. The chopped AC output of the H-bridge passes through a low-pass filter to provide two synchronous 110/120 V AC, 50/60 Hz sinewave outputs that are 180 degrees out of phase. The outputs are combined to provide 120V AC and 240V AC outputs to standard AC outlets. As a safety feature, the inverter output may be inhibited when the vehicle is moving.
Switched reluctance motors are mounted through gearboxes to the driven wheels. The motor receives electrical control from the central computer and its individual motor circuit in the power control module. The motor also has an encoder, such as a Hall sensor or optical sensor, on the rotor that sends a position/speed signal to the central computer in the power control module where it is processed to determine the speed and position of the rotor. Sensorless systems are also well known in the prior art and may be used with this invention. The output shaft of the motor is mechanically directly connected to the input gear in the gearbox.
The preferred embodiment gearbox contains parallel shaft spur gears to provide a 30:1 speed reduction, although different gearing types and reductions may be used for different embodiments. The output shaft of the gearbox is the drive axle for its driven wheel on the vehicle. Torque is increased in the drive axle by the gear ratio in the gearbox.
The power control module contains the central computer, a generator circuit for the generator, and an individual motor circuit for each motor in the system. The central computer accepts speed control signals as varying DC voltage levels from a joystick or a potentiometer mounted on a foot pedal, control panel, steering wheel or a remote location. A digital input signal from a digital device such as another computer or an encoder can also be accepted by the computer for speed control. Pulses from a position/speed sensor on the rotor of the generator and each motor are fed back to the central computer board. For the generator, the position data is used by the central computer and generator circuit for control of the commutation of the phase excitation in the stator winding. For each motor, a similar action occurs with the motor circuit and the position data is also used by the motor circuit and central computer for commutation control in the phase outputs. In an outer (speed) control loop, the stream of pulses is used by the central computer to determine the speed of each motor and to compare the motor speed to the speed set point to determine if a speed correction signal is required to increase or decrease the power signal to that motor. In an inner xe2x80x9cTorquexe2x80x9d (current) control loop, a current signal for each motor is detected and sent back to the central computer where it is compared to a current set point to determine if the current should be increased or decreased to the motor whose current was detected. Each motor in the system is controlled by the appropriate motor circuit in this manner.
In this invention, the drive can operate in a forward direction or a reverse direction. The direction of each motor""s rotation is controlled independently of the other motors in the system. The computer can command a reversal of direction for each wheel through a software generated command or a signal from a reverse/forward switch located on the steering wheel, a control panel, a joystick control, or a remote location. This mode of control allows several different functions to be performed.
The drive configuration with its control scheme in this invention performs the functions of a mechanical differential through a torque (current) control algorithm that is embedded in the software for the central computer. The torque (current) control algorithm balances the torque and changes the speeds of the driven wheels so that the vehicle can make a turn in the same manner that a mechanical differential would allow as defined by the formula:
Speed input to differential=Speed left wheel+Speed right wheel
The motors under most operational conditions will require power from the generator and power control board but when the vehicle is traveling down an incline or decelerating, the motors will regenerate energy back through the power control board and the generator into the source, an internal combustion engine, or other means, which will slow down the vehicle. This regenerative braking is desirable to slow down or stop the vehicle over a reasonable distance.
A two-motor drive is also described with a motor mounted on both the left side and the right side of the generator with a gearbox mounted to the face of each motor opposite the face mounted to the generator. The output shaft of each gearbox is the axle for the wheel on each side of the mower. A metal plate is mounted to the bottom of the assembly tying the two motors/gearboxes and the generator together to stiffen and make a robust assembly.
In this invention, the drive can be configured as an integral assembly as mentioned above or the generator and motor/gearbox wheel units can be mounted on the vehicle separately with electrical power harnesses and signal harnesses run separately between the components in the drive system over the vehicle chassis. A motor circuit controlled by the central computer is required for each motor. One, two, and four-wheel drive configurations can be built in this manner.